The present invention relates to the structure of a semiconductor device and more particularly to the structure of a thyristor.
The withstand voltage of a semiconductor element such as a thyristor having at least one p-n junction is determined from the specific resistance of its high resistance layer. In general, the specific resistance of a semiconductor substrate, from which a semiconductor element is manufactured, is variable inside the semiconductor substrate because of various factors involved in the manufacturing process. Therefore, avalanche breakdown occurs in a small region which is lowest in specific resistance. Accordingly, if a pulsive overvoltage is applied, the small region generates heat which may damage the semiconductor element. Heretofore, in a circuit having a semiconductor element, a ZNR (zinc oxide non-linear resistor) or an avalanche diode was connected in parallel to the semiconductor element, in order to protect the semiconductor element from the overvoltage. For instance, in a high voltage thyristor valve having several hundreds of thyristors connected in series, voltage dividing resistors or anode firing avalanche diodes were connected thereto in order to protect the thyristor valve from a pulsive overvoltage. Thus, the method was disadvantageous in that an intricate protective circuit was required and the device was liable to become bulky.